Bearings may be used in rotating machines to support the rotor in both axial and radial directions. Such bearings may include, for example, lubricated, magnetic, hydrostatic, and gas-dynamic bearings. These bearings generally require a longer rotary shaft and may, in the case of at least magnetic bearings, require complex and costly control systems. In addition to the bearings, the rotating machines, e.g., compressors, typically utilize shaft seals about the rotary shaft to prevent the leakage of process fluid from the housing in which the compressor is disposed. However, the disposition of the shaft seals about the rotary shaft in addition to the aforementioned bearings typically further adds to the required length of the rotary shaft. Generally, a longer rotary shaft increases the weight of the rotor and may reduce the rotor-dynamic performance.
Moreover, in the case of hydrostatic bearings, if the source of pressure for the operation of the hydrostatic bearing is within the turbomachine (i.e., compressor impellers), that pressure may be a function of rotor speed, typically, the square of the speed. Accordingly, the ability of the bearing to support load, e.g., the rotor weight, drops-off rapidly with speed as the pressure differential across the bearing decays, and the bearing may cease to operate when the load capacity falls below the local weight of the shaft system. This may happen at a relatively high speed because the pressure differential falls faster than rotor speed.
In such cases, a system such as a passive permanent magnet system may be utilized to support some amount of the rotor load and allow the bearing to operate at a lower speed, that is, with a reduced pressure differential. However, the passive permanent magnet system may not have the load capacity necessary to support the rotor down to standstill. Accordingly, additional support may be provided by stationary pads that tolerate some rubbing of the rotor, or by an auxiliary bearing system, such as a type employed by active magnetic bearing-supported machinery.
However, the utilization of auxiliary bearing systems designed for active magnetic bearing-supported machinery may present certain challenges as hydrostatic bearings operate on different principles and thus require different operation parameters. In the operation of active magnetic bearings, the rotor is levitated at zero speed. In scenarios where the auxiliary bearings are utilized, contact generally is made with the auxiliary bearings at operating speed as the rotor suddenly drops onto the auxiliary bearings with some impact force when the magnetic bearing fails. Thus, for magnetic bearing service, auxiliary bearing systems are typically used only on rare occasions, are subject to severe service when they are needed, and generally are designed with a limited service life.
With respect to hydrostatic bearings, and in the case of a horizontal rotor system, the auxiliary bearings generally support the rotor on every startup until the speed reaches a level such that the pressure differential builds and the gas bearings may take over support of the load. Furthermore, the auxiliary bearings must generally keep the center of the rotor close to the center of the bearing at all times, as the journal in an axially-fed hydrostatic bearing system does not “lift-off” as the pressure builds. Typically, the bearings are statically unstable at high eccentricity ratios and are incapable of generating sufficient lift. In addition to the foregoing, the auxiliary bearings also must generally support the rotor on every shutdown as well; however, the contact between rotor and auxiliary bearing typically occurs gradually and at a speed lower than full speed, so the duty on the bearing may be much less severe. In the case of a vertically mounted rotor, the radial bearings generally have no gravity load to locate the rotor at lower speeds when the pressure differential becomes small, and a design of the auxiliary bearing that centers the rotor and resists whirl is very advantageous.
Thus, in conjunction with a hydrostatic bearing, a conventional approach has been the utilization of an auxiliary bearing having a concentric rolling element bearing, such that a rotor positioned within the clearance of the concentric rolling element bearing, upon failure of the primary bearing, falls onto the inner surface of the inner ring (or a separate insert) of the concentric rolling element bearing. However, although such an auxiliary bearing may provide support, stiffness and damping in the vertical axis, it provides essentially no support, stiffness or damping in the horizontal direction. Therefore, such an auxiliary bearing is very poor at positioning the rotor horizontally. For a vertical rotor, a radial bearing of this type has no effect on the rotor until the orbit is so large that it touches the inner surface of the bearing. This results in high vibration and poor centering of the rotor.
There is a need, therefore, for an auxiliary bearing system capable of providing stable support when the bearing and seal combination cannot support the rotor loads independently and further capable of providing improved stiffness and damping in the horizontal direction for a horizontally-oriented rotor or providing stiffness and damping with reduced rotor motion for a vertically-oriented rotor. Further, there is a need for an auxiliary bearing system capable of hundreds of start/stop cycles without maintenance.